CVD of Al2 O3 layers on cutting inserts

ABSTRACT

The present invention relates to a method for coating a cutting insert having at least one layer of a carbide, carbonitride, oxynitride, oxycarbide, oxycarbonitride or nitride of one metal selected form the groups IVB, VB or VIB of the Periodic Table, the method comprising subjecting the insert to chemical vapor deposition and coating the insert with an epitaxial kappa-Al 2  O 3  layer by the hydrolysis reaction of AlCl 3 , H 2  and CO 2 , the hydrolysis reaction being carried out while maintaining the content of water vapor in the gas phase during nucleation of Al 2  O 3  at less than 5 ppm.

This application is a divisional of Application Ser. No. 07/717,079, filed Jun. 18, 1991, now abandoned, which is a continuation of Ser. No. 07/539,567 filed on Jun. 15, 1990, now U.S. Pat. No. 5,071,696.

BACKGROUND OF THE INVENTION

The present invention relates to a coated cutting insert for chipforming machining.

The Chemical Vapor Deposition (CVD) of alumina on cutting tools has been an industrial practice for more than 15 years. The wear properties of Al₂ O₃ as well as of TiC and TiN have been discussed extensively in the literature.

The CVD technique has also been used to produce coatings of other metal oxides, carbides and nitrides with the metal selected from the transition metals of the groups IVB, VB and VIB of the Periodic Table, silicon, boron and aluminum. Many of these compounds have found practical applications as wear resistant or protective coatings, but few have received as much attention as TiC, TiN and Al₂ O₃.

Initially, coated tools were intended for turning applications, but today tools designed for milling as well as drilling applications, are coated. Improvements of the bonding to the substrate (generally a cemented carbide) and between different coating materials have resulted in a plurality of coating combinations with double-, triple- and multi-layer structures.

The reaction mechanisms occurring during the CVD of Al₂ O₃ have been analyzed, but little has been mentioned about the stability and microstructure of the deposited Al₂ O₃ phases and how the formation of these phases depends on the deposition process.

Al₂ O₃ crystallizes in several different phases of which the alpha-structure (corundum) is the thermodynamically stable phase at typical deposition temperatures. The metastable kappa-phase is the second most commonly occurring modification in CVD-Al₂ O₃. Other infrequently occurring types are theta-, gamma- and delta-Al₂ O₃.

In commercial tools, Al₂ O₃ is always applied on TiC-coated cemented carbide (see, e.g., U.S. Pat. No. 3,837,896, now U.S. Pat. No. Re. 29,420) and therefore the interface reactions of the TiC-surface are of particular importance. The TiC-layer should also be understood to include those layers having the formula TiC_(x) N_(y) O_(z) in which the C in TiC is completely or partly substituted by oxygen and/or nitrogen. In the system TiC_(x) N_(y) O_(x) there is 100% miscibility. There can be one or more layers of this kind. Similar relationships also exist within other systems, e.g., Zr--C--N--O.

The purpose of the present invention has been to obtain such an Al₂ O₃ -layer with the correct crystallography, microstructure and morphology and under nucleation conditions such that the desired Al₂ O₃ -phases will be stabilized.

A typical surface structure of a CVD-Al₂ O₃ coating contains coarse-grained islands of alpha-Al₂ O₃, the first Al₂ O₃ phase to nucleate, surrounded by much more fine-grained kappa-Al₂ O₃ areas. When greater amounts of such alpha-Al₂ O₃ exist the individual islands join together. The ratio of coarse-grained/fine-grained areas can vary over a broad range.

Also, the alpha/kappa ratio measured by X-ray diffraction will vary over a wide range. A higher ratio is obtained at long coating times and after further heat treatment. After about 3 hours heat treatment at 1000° C., essentially all the thermodynamically metastable kappa-Al₂ O₃ is transformed to stable alpha-Al₂ O₃. It is important to note that the transformation kappa→alpha takes place without any great changes in surface morphology. The alpha-Al₂ O₃ formed from kappa-Al₂ O₃ is fine-grained.

The kinetics behind the phase transformation of metastable kappa-Al₂ O₃ to stable alpha-Al₂ O₃ is not clear. It should be noted, however, that only small rearrangements of the close-packed oxygen layers that are common to both structures, are needed to convert kappa- to alpha-Al₂ O₃.

In U.S. Pat. No. 4,180,400 (now U.S. Pat. No. Re. 31,526) a method of making kappa-Al₂ O₃ has been proposed being based essentially on the fact that an addition of TiCl₄ to the gas mixture results in kappa-Al₂ O₃ formation. Any kappa-phase, formed under these conditions, does not have an epitaxial relationship with the layer below a thick Al₂ O₃ -layer, however. (A thick Al₂ O₃ -layer is more than 2 μm Al₂ O₃).

The microstructure of an alpha-Al₂ O₃ coating initially nucleated as alpha-Al₂ O₃ is characterized by a grain size of about 0.5-5 μm, preferably 0.5-2 μm, and the presence of pores. Contrary to this, a kappa-Al₂ O₃ coating is essentially pore-free and has a much smaller grain size of about 0.5-2 μm, preferably 0.05-1 μm, usually about 0.2-0.5 μm.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to alleviate or substantially reduce the problems of the prior art.

It is also an object of the present invention to provide an alumina coating on a metal carbide, nitride, carbonitride, oxynitride, and oxycarbides having an improved adhesion.

It is a further object of the invention to provide a cutting tool having multiple coatings including at least one coating of alumina in contact with a metal carbide, nitride, carbonitride, oxynitride or oxycarbonitride having improved adhesion between the layers.

In one aspect of the present invention, there is provided a body coated with at least one layer of Al₂ O₃ and at least one layer of a carbide, nitride, carbonitride, oxynitride or oxycarbonitride of a metal selected from the group consisting of metals in the Groups IVB, VB and VIB of the periodic table, B, Al and Si and mixtures thereof wherein the Al₂ O₃ layer in contact with the said metal carbide, nitride, carbonitride or oxycarbonitride layer comprises epitaxial kappa-alumina or theta-alumina.

In another aspect of the present invention, there is provided a metal cutting insert comprising a cemented carbide substrate coated with at least one layer of TiC and at least one adjacent layer of alpha Al₂ O₃, at least some of the alpha-Al₂ O₃ having been formed by the transformation of epitaxial kappa- or theta-Al₂ O₃.

In still another aspect of the present invention, there is provided a method for coating a body by chemical vapor deposition of Al₂ O₃ by the hydrolysis reaction of AlCl₃, H₂ and water, the improvement which comprises maintaining the content of water vapor during nucleation of the Al₂ O₃ at less than 5 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an Al₂ O₃ -surface consisting of initially nucleated alpha- (coarse-grained) and kappa- (fine-grained) structure; and

FIG. 2 shows the same microstructural feature, but after 4 hours heat treatment, by which a microstructure of alpha-Al₂ O₃ of fine-grained type has been obtained by transformation of kappa-Al₂ O₃ produced according to the invention, mixed with initially formed alpha-Al₂ O₃.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has now been found that poor adhesion between the Al₂ O₃ -layer and the underlying TiC-layer is obtained if the Al₂ O₃ in direct contact with the TiC-layer has the alpha-Al₂ O₃ crystal structure at the time when it is nucleated. In such a case, considerable porosity exists between the two layers. If, on the other hand, kappa- or theta-Al₂ O₃ is in direct contact with the TiC-layer, the transition is pore-free and good adhesion is obtained. In the latter case a direct lattice relationship exists between the orientations of the Al₂ O₃ and the TiC-layers, i.e., the Al₂ O₃ -layer grows epitaxially. The following relationships exist between TiC/kappa-Al₂ O₃ :

(111) TiC//(0001) kappa-Al₂ O₃

[110] TiC//[1010 ] kappa-Al₂ O₃

and between TiC/theta-Al₂ O₃ :

(111) TiC//(310) theta-Al₂ O₃

[110] TiC//[001] theta-Al₂ O₃

Both of these can be interpreted as indicating that close-packed atom planes in the Al₂ O₃ - and TiC-phases are in contact and thus the adhesion across the interface between Al₂ O₃ and TiC will be maximized. Similar epitaxial relationships should also exist between Al₂ O₃ and metal systems related with TiC, e.g., ZrC and for other carbides, carbonitrides, oxynitrides, oxycarbides, oxycarbonitrides and nitrides of metals in the groups IVB, VB and VIB of the Periodic Table and for B, Al and Si and should maximize adhesion between the phases on either side of the interface.

The process conditions governing the first nucleation event are thus of great importance. The oxidation state of the TiC-surface is decisive for the kind of nuclei which are to be formed. If the TiC-surface is (intentionally) oxidized only alpha-Al₂ O₃ is formed. Simple calculations show that the presence of water in H₂ carrier gas has a dramatic effect on the oxidation state. Already at 5 ppm water in H₂ at 1000° C. and 560 mbar the TiC surface oxidizes to Ti₂ O₃. At higher water contents (20-30 ppm) Ti₃ O₅ is formed. It should be observed that the formation of titanium oxide (Ti₂ O₃, Ti₃ O₅) involves a volume increase of 25-30%. Also, other oxygen containing compounds than water in the feed-gas can cause oxidation.

The water gas concentration in the reactor during Al₂ O₃ coating at typical conditions for good adhesion is very small and all water is used during the hydrolysis reaction with AlCl₃. For a typical reactor, the water content is 0.01-0.1 ppm. The water concentration varies, however, greatly during the nucleation event and can locally reach 1000-2000 ppm. It is thus possible to oxidize the TiC surface during the Al₂ O₃ nucleation stage. Another contribution to the oxidation of the TiC-surface comes from the heterogeneous decomposition of CO₂, in which surface adsorbed mono-atomic oxygen can oxidize the TiC-surface.

It is surprising that such low-oxidizing conditions would be necessary to obtain a well-adherent oxide coating (Al₂ O₃). The prior art even recommends the use of oxidized surfaces to form well-adherent dense Al₂ O₃ -coatings (see, e.g., U.S. Pat. Nos. 3,736,107; 3,837,896; 4,018,631 or 4,608,098).

It has thus been found that no well-defined lattice-orientation relationship exists between TiC and alpha-Al₂ O₃, which has been nucleated on TiC from gas phase. Instead, the TiC-Al₂ O₃ does not take place on TiC but on a thin oxide film of e.g., Ti₂ O₃, Ti₃ O₅ or TiO₂ what thus makes the formation of an epitaxial relationship with the underlying TiC-surface impossible. However, because there is no trace of any intermediate layer of Ti₂ O₃, Ti₃ O₅ or TiO₂ in the final product it must be assumed that a transformation to TiO, TiCO or TiC has happened during the relatively extended coating period. The volume contraction (25-30%) which accompanies the phase transformation Ti₃ O₅ or Ti₂ O₃ to TiO would explain the observed interfacial porosity.

Contrary to this, kappa-Al₂ O₃ or theta-Al₂ O₃ is nucleated directly on non-oxidized or relatively weakly oxidized (up to highest Ti₂ O₃) TiC-surfaces, which results in an epitaxial relationship between the TiC- and the Al₂ O₃ -layers and, as then, no reduction of Ti₂ O₃ etc. to TiO occurs, thus in the absence of porosity.

The alpha-Al₂ O₃ is, as mentioned, the stable structure and has a higher density than kappa-Al₂ O₃. If, during the nucleation, kappa-Al₂ O₃ is formed then the resulting layer will have a more fine-grained and uniform structure than, in particular, a mixture of kappa- and alpha-Al₂ O₃. If only or almost only alpha-Al₂ O₃ is obtained, it takes place in connection with the reduction of higher titanium oxides (e.g., TiO₂ Ti₃ O₅) and porosity will be formed.

FIG. 1 shows an Al₂ O₃ -surface consisting of initially nucleated alpha- (coarse-grained) and kappa- (fine-grained) structure. As may be seen, the majority of the Al₂ O₃ is nucleated in the kappa form while the remainder is initially in the alpha form.

FIG. 2 shows the same microstructural feature, but after 4 hours heat treatment, by which a microstructure of alpha-Al₂ O₃ of fine-grained type has been obtained by transformation of kappa-Al₂ O₃ produced according to the invention, mixed with initially formed alpha-Al₂ O₃.

According to the invention there is thus now available a body such as, e.g., a cutting insert for chipforming machining with a substrate of cemented carbide as known in the art and as commercially available, coated with at least one layer of Al₂ O₃ and one layer of TiC in which the Al₂ O₃ -layer is in epitaxial contact with an adjacent TiC-layer and comprises the form of kappa- or theta-Al₂ O₃. In general, the coated body is treated, for example, by subsequent heat treatment, so that at least 90%, preferably at least 98%, of the initially nucleated kappa- or theta-Al₂ O₃ is transformed to alpha-Al₂ O₃. By this measure, a dense and wear resistant alpha-Al₂ O₃ layer is obtained which is fine-grained and has a good bond to the underlying TIC-, TiN- or TiO-layer. The fine grain size and the good bond are caused by the excellent nucleation properties of the initially nucleated kappa-Al₂ O₃ -layer. In said respect, the invention can be described as alpha-Al₂ O₃ with kappa-Al₂ O₃ -morphology. That is, the occurrence of alpha-Al₂ O₃ can only be verified by X-ray diffraction or similar methods.

The invention also relates to a method for obtaining Al₂ O₃ -layers with good adherence to the substrate (including the TiC, TiN, etc. interlayer). By said method, the coating shall be performed so that oxidization of the surface of the TiC-layer is avoided at the transition to the Al₂ O₃ -coating process by keeping the water vapor concentration at the nucleation below 5 ppm, preferably below 0.5 ppm, most preferably from 0.01-0.1 ppm. In this context, it should be noted that the interlayer must have a thickness and such properties that metals (e.g., Co) catalyzing the CO₂ decomposition to oxygen and CO cannot penetrate the interlayer and cause excessive oxidation of the surface on which Al₂ O₃ is to be nucleated. The coated body can then alternatively be heat treated for 0.3-10, preferably 1-4, hours, at a temperature of 900°-1100° C. preferably in a protective gas atmosphere as known to the skilled artisan.

By double layer is also meant known layers of current multitype inner as well as outer layers. Also, when, e.g., TiC or TiN is deposited on Al₂ O₃ the invention can be applied.

The invention is additionally illustrated in connection with the following Examples which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the Examples.

EXAMPLE 1

Cutting inserts were coated under the same conditions as in Example 1 in U.S. Pat. No. 3,837,896 by which a 3 μm thick layer was obtained by suitably sparse packing and hydrolysis reaction to AlCl₃, H₂ and CO₂. The water concentration was kept to such a low level before and during the Al₂ O₃ -nucleation that mainly kappa-Al₂ O₃ was nucleated in contact with TiC, cutting insert A.

The cutting inserts were then tested with respect to edge line flaking with the results given below.

The flaking test was performed as a facing operation of a low carbon steel test piece with intermittent cuts obtained by preformed slits cut into the test piece. Since low carbon steels vary in composition and other properties the configuration of the slits and cutting data must be adjusted by performing preliminary tests enabling final best conditions to be obtained. The results of the cutting test is expressed as the ratio of flaking length of the edge line and the total edge line in the cutting operation.

    ______________________________________                                         Test   % alpha-Al.sub.2 O.sub.3                                                                         flaking                                               ______________________________________                                         A      15                70 (acc. to the inv.)                                 B      90                90 (known techn.)                                     ______________________________________                                    

Testing of the wear resistance showed that the variant B, in addition, had inferior face wear resistance and shorter life.

EXAMPLE 2

Cutting inserts coated under the same conditions as in Example 1 and in which the coating thus consisted essentially of about 3 μm kappa-Al₂ O₃ were heat treated at 1020 C. in a hydrogen gas atmosphere and the kappa-phase was successively transformed to alpha-phase.

The inserts were then tested with respect to edge line flaking with results below:

    ______________________________________                                         Test   Heat treatment time, h                                                                          % alpha-Al.sub.2 O.sub.3                                                                   flaking                                    ______________________________________                                         C      8                100         65                                         D      2                35          50                                         ______________________________________                                    

Testing of the wear resistance showed that the variant D had inferior face wear resistance and the shortest life of the inserts C and D.

EXAMPLE 3

Cutting inserts were coated in accordance with Example 1 in U.S. Pat. No. 3,837,896 at which 1 μm Al₂ O₃ -layer was obtained by a dense packing of inserts and supporting details. The coated inserts had a high carbon content so that about 6 μm thick TiC-layer was formed. Inserts were taken out from different parts of the coating charge. One group of inserts had kappa-Al₂ O₃ in contact with the TiC-layer. These inserts showed in an intermittent turning test no tendency for the Al₂ O₃ -layers to "flake" from the TiC-layer. Another group of inserts with almost only alpha-Al₂ O₃ in contact with TiC flaked to about 45% of the surface exposed to the chip.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing application. The invention which is intended to be protect herein should not, however, be construed as limited to the particular forms disclosed, as these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the present invention. Accordingly, the foregoing detailed description should be considered exemplary in nature and not limited to the scope and spirit of the invention as set forth in the appended claims. 

What is claimed is:
 1. A method of coating a body adapted for a cutting insert having at least one layer of a carbide, carbonitride, oxynitride, oxycarbide, oxycarbonitride or nitride of one metal selected from the groups IVB, VB, or VIB of the Periodic Table, the method comprising subjecting the body to chemical vapor deposition and coating the body with an epitaxial kappa-Al₂ O₃ layer by the hydrolysis reaction of AlCl₃, H₂ and CO₂, the hydrolysis reaction being carried out while maintaining the content of water vapor in the as phase during nucleation of Al₂ O₃ at less than 5 ppm.
 2. The method of claim 1, wherein the water vapor during the nucleation is less than 0.5 ppm.
 3. The method of claim 1, wherein after coating, the body is heat treated for 0.3-10 hours at 900°-1100° C. in a protective-gas atmosphere.
 4. The method of claim 1, wherein the body includes a layer of titanium carbide and the Al₂ O₃ layer is deposited in contact with the titanium carbide layer during the coating step.
 5. The method of claim 4, wherein the body comprises a cemented carbide substrate coated with the titanium carbide layer.
 6. The method of claim 4, wherein the following lattice orientation relationship exists between the kappa-Al₂ O₃ and titanium carbide layers:(111)TiC//(0001)kappa-Al₂ O₃ [110] TiC//[0011] kappa-Al₂ O₃.
 7. The method of claim 1, wherein the body is a metal cutting insert.
 8. The method of claim 4, wherein oxidation of the titanium carbide layer is prevented during the coating step.
 9. The method of claim 1, further comprising heat treating the coated body and transforming the kappa-Al₂ O₃ layer to an alpha-Al₂ O₃ layer.
 10. The method of claim 1, wherein the kappa-Al₂ O₃ layer is essentially pore free.
 11. The method of claim 1, wherein the body includes a layer of titanium carbonitride and the Al₂ O₃ layer is deposited in contact with the titanium carbonitride layer during the coating step.
 12. A method of coating a cutting insert having at least one layer of a carbide, carbonitride, oxynitride, oxycarbide, oxycarbonitride or nitride of one metal selected from the groups IVB, VB or VIB of the Periodic Table, the method comprising subjecting the insert to chemical vapor deposition and coating the insert with an epitaxial kappa-Al₂ O₃ layer by the hydrolysis reaction of AlCl₃, H₂ and CO₂, the hydrolysis reaction being carried out while maintaining the content of water vapor in the gas phase during nucleation of Al₂ O₃ at less than 5 ppm.
 13. The method of claim 12, wherein the insert includes a layer of titanium carbide and the Al₂ O₃ layer is deposited in contact with the titanium carbide layer during the coating step.
 14. The method of claim 13, wherein oxidation of the titanium carbide layer is prevented during the coating step.
 15. The method of claim 13, wherein the following lattice orientation relationship exists between the kappa-Al₂ O₃ and titanium carbide layers:(111 ) TiC//(0001) kappa-Al₂ O₃ [110] TiC//[0010] kappa-Al₂ O₃.
 16. The method of claim 12, further comprising heat treating the insert and transforming the kappa-Al₂ O₃ layer to an alpha-Al₂ O₃ layer.
 17. The method of claim 12, wherein the kappa-Al₂ O₃ layer is essentially pore free.
 18. The method of claim 12, wherein the insert includes a layer of titanium carbonitride and the Al₂ O₃ layer is deposited in contact with the titanium carbonitride layer during the coating step.
 19. The method of claim 12, wherein the water vapor content during the nucleation of Al₂ O₃ is less than 0.5 ppm. 